1. Field of the Invention
The invention relates to a method of producing inert, catalytically active or gas sensitive ceramic layers for gas sensors, particularly for potentiometric or resistive gas sensors, preferably with a volume effect, such ceramic layers being produced from a paste composed of a powdered basic ceramic material and an organic basic material for the paste.
2. Technology Review
The concentration of gases, particularly the concentration of oxygen and reducing gases, is measured to an increasing degree by means of suitable sensors. Perovskite, particularly mixed titanium oxides, among others, have been employed as semiconductors for such sensors. In dependence on the partial pressure of the gases to be measured and the sensor temperature, these semiconductors exhibit a great change in electrical resistance. If the temperature is known or is compensated, it is possible to utilize the measurement of the semiconductor resistance to determine the partial pressure of these gases to be measured.
Such sensors can be miniaturized and are extremely resistant to external influences because of the chemical stability and mechanical hardness of the semiconductor ceramics active in the measurements and are therefore almost ideally suited for measuring tasks in difficultly accessible, rough environments.
A particular requirement for gas sensors has developed in the automobile industry because, for reasons of environmental protection, it is necessary to set the fuel/air mixture in every driving state as accurately as possible. However, in this connection the requirements to be met by the sensors are especially high; particularly for use in high revolution internal-combustion engines involving up to 6000 rpm the response time of the sensors must be extremely short.
Semiconductors for gas sensors are presently produced by three different methods:
1. in a thin-film process; PA0 2. by sintering of ceramic pellets; PA0 3. by manually applying the paste with the aid of a brush.
The production of semiconductors for gas sensors in a thin-film process by way of reactive high frequency sputtering is disclosed, for example, in European Patent No. 0,131,731.
This method makes it possible to produce extremely thin gas sensors having a very small surface area.
Sensors produced according to this method exhibit a very "fast" response behavior, i.e. the change in resistance takes place with a very short time delay compared to the change in concentration of the gas being measured.
However, this manufacturing method is very complicated and thus expensive because it must be performed in a high vacuum and because the sputtered-on mixed oxide layers must be subjected to further treatment.
Due to the short free path lengths of the particles to be applied during sputtering and the resulting collision processes, no exact geometric structures can be produced even if templates are employed.
Moreover, doping the semiconductors in order to optimize their characteristics is possible only very conditionally since the number of elements that can be applied simultaneously by sputtering is limited. For these reasons, the thin-film technology is acceptable only in very specific fields.
Many sensors are therefore produced as ceramics by way of sintering. Such sintered sensors are disclosed, for example, in U.S. Pat. Nos. 4,044,601, 3,953,173 and 4,454,494.
The thickness of the sintered ceramic layer is at least 20 .mu.m, usually, however, 500 .mu.m. The speed of response of such sensors is noticeably poorer compared to those produced in a thin-film process (sputtering) because the time required to respond to changes in the concentration of the gas being measured is proportional to the square of the layer thickness and therefore increases considerably with increasing layer thickness.
DE-OS 3,024,449 discloses that semiconductors for gas sensors can be produced by manually applying a paste containing the semiconductor material. The layers are then more than 20 .mu.m thick. The method is difficult to reproduce, requires subsequent treatment and is not suitable for the mass production of gas sensors.
For measuring technology reasons it may be of advantage to coat the gas sensitive semiconductor layer with a chemically inert, not gas sensitive intermediate layer and a catalytically active layer thereabove.